This invention relates to reducing xylene loss in a continuous process for isomerizing a xylene-containing feed to a mixture rich in p-xylene in the presence of hydrogen over an isomerization catalyst which does not effectively reduce olefins produced during the isomerization process and, more particularly, relates to reducing xylene loss in a continuous process for isomerizing a xylene-containing feed to a mixture rich in p-xylene in the presence of hydrogen over a transalkylation-type of isomerization catalyst in which the small amount of low molecular weight olefinic hydrocarbons produced during isomerization are removed with a hydrogenation catalyst separate from the isomerization catalyst.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by isomerization followed by, for example, lower-temperature crystallization of the paraxylene with recycle of the crystallizer liquid phase to the isomerizer. Principal raw materials are catalytically reformed naphthas and petroleum distillates. The fractions from these sources that contain the C.sub.8 aromatics vary quite widely in composition but will usually contain 10 to 35 wt. % ethylbenzene and up to about 10 wt. % primarily C.sub.9 paraffins and naphthenes with the remainder being primarily xylenes divided approximately 50 wt. % meta, and 25 wt. % each of the ortho and para isomers. The primarily C.sub.9 paraffins and naphthenes can be removed substantially by extraction to produce what are termed "extracted" xylene feeds. Feeds that do not have the primarily C.sub.9 paraffins and naphthenes removed by extraction are termed "unextracted" xylene feeds.
In the typical commercial process, isomerization of the xylene-containing feed takes place in the presence of hydrogen, and since little hydrogen is consumed in a once-through operation, a separation of the hydrogen and "light ends" is made after isomerization and returned to the isomerizer feed in the gas recycle stream.
Xylene isomerization catalysts can be classified into three types based upon the manner in which they convert ethylbenzene: (1) naphthene pool catalysts, (2) transalkylation catalysts, and (3) hydrodeethylation catalysts.
Naphthene pool catalysts are capable of converting a portion of the ethylbenzene to xylenes via naphthene intermediates. These catalysts contain a strong hydrogenation function, such as platinum, and an acid function, such as chlorided alumina, amorphous silica-alumina, or a molecular sieve. The role of the hydrogenation function in these catalysts is to hydrogenate the C.sub.8 aromatics to establish essentially equilibrium between the C.sub.8 aromatics and the C.sub.8 cyclohexanes. The acid function interconverts ethylcyclohexane and the dimethylcyclohexanes via cyclopentane intermediates. These C.sub.8 cycloparaffins form the so-called naphthene pool.
It is necessary to operate naphthene pool catalysts at conditions that allow the formation of a sizable naphthene pool to allow efficient conversion of ethylbenzene to xylenes. Unfortunately, naphthenes can crack on the acid function of the catalyst, and the rate of cracking increases with the size of the naphthene pool. Naphthene cracking leads to high xylene loss, and the byproducts produced by naphthene cracking are low-valued paraffins. Thus, naphthene pool catalysts are generally less economic than the transalkylation-type and hydrode-ethylation-type catalysts. Because of the strong hydrogenation character of this type of catalyst, any alkenes produced during isomerization would be reduced to alkanes.
The transalkylation catalysts generally contain a shape-selective molecular sieve. A shape-selective catalyst is one that prevents some reactions from occurring based on the size of the reactants, products, or intermediates involved. In the case of common transalkylation catalysts, the molecular sieve contains pores that are apparently large enough to allow ethyl transfer to occur via a dealkylation/realkylation mechanism, but small enough to substantially suppress methyl transfer which requires the formation of a bulky biphenylalkane intermediate. The ability of transalkylation catalyst to catalyze ethyl transfer while suppressing methyl transfer allows these catalysts to convert ethylbenzene while minimizing xylene loss via xylene disproportionation. The small amounts of lower alkenes such as ethylene, propylene, etc., produced during isomerization are not hydrogenated to alkanes and, they buildup in the system because they are recycled with the hydrogen in the recycle gas stream.
When ethyl transfer occurs primarily by dealkylation/realkylation, it is possible to intercept and hydrogenate the ethylene intermediate involved with this mechanism of ethyl transfer by adding a hydrogenation function to the catalyst. The primary route for converting ethylbenzene then becomes hydrodeethylation, which is the conversion of ethylbenzene to benzene and ethane. It is desirable to selectively hydrogenate the ethylene intermediate without hydrogenating aromatics (without establishing a naphthene pool) to prevent the cracking of the naphthenes that occurs over the acid function of the catalyst. Commercial hydrodeethylation catalysts selectively hydrogenate ethylene without substantial hydrogenation of aromatics at reported commercial conditions. At these same conditions, a small amount of certain impregnated metal compounds causes substantial hydrogenation of aromatics reducing the amount of p-xylene produced by the process.
In order to form a hydrodeethylation catalyst, it is essential to use an acidic component that behaves as a shape selective catalyst, i.e., one that suppresses the formation of the bulky biphenylalkane intermediate required for transmethylation, because transethylation can occur via a similar intermediate. For catalysts with pores large enough to allow the formation of these biphenylalkane intermediates, transethylation appears to occur primarily via these intermediates. In this case, ethylene is not an intermediate for transethylation, and the addition of a hydrogenation component cannot produce a hydrodeethylation catalyst.
When using a transalkylation-type isomerization catalyst in the typical commercial process, lower molecular weight olefins are produced in the isomerization and are returned to the isomerization reactor, after removal from the isomerization product, in the gas recycle stream where they can build up in the process. These lower molecular weight olefinic compounds, such as ethylene, propylene, and the like, can react with the xylene reducing the overall yield of p-xylene in the process. They can also lead to coke buildup on the isomerization catalyst which increases pressure drop in the reactor, decreases catalyst lifetime, and means the process temperature must be increased to maintain constant catalyst activity.
Now it has been found for xylene isomerization using a catalyst unable to substantially reduce the lower molecular weight olefins produced during isomerization, that use of a hydrogenation catalyst in the process to convert such olefins can substantially reduce the xylene loss leading to a greater overall p-xylene yield, and can also lead to lessen catalyst coking and longer catalyst lifetime.